This invention relates in general to quadrupole mass spectrometers and more particularly to a Fourier transform quadrupole mass spectrometer for simultaneously analyzing a mass range of ions.
The use of Fourier analysis in mass spectrometry is well known. The primary application of Fourier analysis methods in mass spectrometry has been in the area of Ion Cyclotron Resonance. The basic method of this technique was described in U.S. Pat. No. 3,937,955 entitled "Fourier Transform In Cyclotron Resonance Spectroscopy Method and Apparatus". More recently Knorr, Ajami & Chatfield (Anal. Chem. 1986, 58, 690-694) have described a Fourier transform method involving time of flight mass spectrometry.
The apparatus and method of mass analysis described herein is an enhancement of the technique that is referred to in the literature relating to quadrupole mass spectrometry as "mass selective detection". There are very important differences between the prior art method and apparatus and the method and apparatus of the present invention. The prior art Fourier Transform technique involves analyzing the orbital frequency of ions constrained in a large magnetic field whereas the present method and apparatus involves measuring a component frequency of the oscillatory motion of ions immersed in a radio frequency quadrupole electric field.
The earliest description of the use of the mass selective detection technique in radio frequency fields is in U.S. Pat. No. 2,939,952 which teaches both a radio frequency quadrupole mass filter and a radio frequency quadrupole ion trap. Fischer (Z. Phys., 156 (1959) 26) and Rettinghaus (Z. Angew Phys., 22 (1967) 321) built quadrupole ion trap mass analyzers based on the Paul and Stienwedel concept. A discussion of the principles of operation of the quadrupole ion trap can be found in the book "Quadrupole Mass Spectrometry and its Applications" edited by Peter Dawson, pages 49-52 and 184-188. Radio frequency quadrupole ion traps are also discussed in chapter 4, pages 39-49 of the Book Dynamic Mass Spectrometry, Vol. 4, edited by D. Price and J. F. J. Todd.
In addition to the mass analyzer based on mass selective detection two other mass analyzers have been described using RF quadrupole ion traps. Dawson and Whetten (U.S. Pat. No. 3,537,939) described a radio frequency (RF) quadrupole ion trap mass analysis method based on mass selective storage. Stafford, et al. (U.S. Pat. No. 4,540,884) described an RF quadrupole ion trap mass analysis method based on mass selective instability.